Switching Power Converters have for many years served as a viable means for electrical energy conversion. By using non-power-dissipative semi-conductor switching elements relatively high power conversion efficiencies have been achieved. Unfortunately, although semiconductor devices are operated in a manner similar to that of switches, undesirable energy dissipation internal to these conventional devices occurs during turn-on and turn-off transitions. Such losses are due to the simultaneous existence of voltage across and current through the semiconductor devices during commutation. Because these losses occur at each switch transition, high frequency operation correspondingly yields low power conversion efficiencies.
Since higher switching frequencies generally result in smaller reactive components and improved dynamic performance, mechanisms for minimizing switching losses have long been sought after. For example, in conventional Pulse-Width-Modulated (PWM) switch-mode converters, energy recovery snubbers have been used to "soften" the switching of semiconductor devices. A technique known as "soft-switching" has also been implemented in switching power converters. This conventional technique seeks to eliminate switching losses by altering the switching conditions in such a way that the switch current or switch voltage is zero at the time of commutation.
In this way "Zero-Current-Switching" (ZCS) or "Zero-Voltage-Switching" (ZVS) respectively, is attempted. To implement this switching mechanism, an L-C network is added around the switch so that the switch current or switch voltage may be kept at a constant zero value during switch commutation. Conventional switch-mode converters using this type of soft-switch are known as Quasi-Resonant Converters.
ZCS may be attempted when an inductor is placed in series with the semiconductor switch FIG. 1a. Since the energy stored in an inductor cannot change instantaneously, therefor neither can the current through it change instantaneously. If energy resonates between the inductor and the capacitor when the switch is on, then the switch may be opened losslessly (in theory) at a time when the inductor has dumped all of its energy to the capacitor. Once the switch is open, the inductor current remains zero, and the switch can turn on with zero-current through it.
ZVS may be attempted when a capacitor is placed in parallel with the semiconductor switch (FIG. 2b and 2c). Since energy stored in the capacitor cannot change instantaneously, therefor neither can the voltage across it change instantaneously. If energy resonates between the capacitor and the inductor when the switch is off, then the switch may be closed losslessly (in theory) at a time when the capacitor has dumped all of its energy to the inductor. Once the switch is closed, the capacitor is shorted and its voltage remains zero, thus allowing zero-voltage-turn-off for the switch.
In the conventional soft-switching techniques discussed above, the switch element must commute off (for ZCS) or on (for ZVS) at the instant in time when the inductor current or capacitor voltage crosses zero, respectively. This switching is extremely difficult to implement in practice since the switch current or switch voltage must be sensed by a control circuit which must in turn react quickly enough to actively commute the switch before the zero-switching opportunity has passed. Switches can be designed to commute to a first state actively, but commute to a second state either actively or passively. For "Zero Current" (ZC) switching, a switch is needed which will passively (i.e. naturally) turns off. For "Zero Voltage" (ZV) switching, a switch is needed which will passively turn on.
Diodes have long been conventionally used as passive switches, turning on when forward biased, and turning off when reverse biased. They may be connected to an active switch in one of two ways: in series, or in parallel. An active switch with a series diode is referred to as an uni-directional switch since it can conduct current in only one direction FIG. 1b. If the active switch is connected with a parallel diode, it is termed a bi-directional switch since current may flow in two possible directions FIG. 1c. Conventional ZCS converters possessing these types of switches are said to operate in a half-wave mode and full-wave mode, respectively. Full-wave mode refers to the inductor current resonating to negative as well as positive values, and half-wave mode refers to the inductor current flowing in only one direction.
Both uni-directional and bi-directional switches can turn off passively, however an uni-directional switch turns off passively when the inductor current tries to flow negatively through the switch, whereas a bi-directional switch turns off passively when the inductor current tries to flow positively through the switch. Here switch refers to the composite switch formed by the active semiconductor switch and the passive diode switch. For a uni-directional switch, the active switch must remain on until the series diode blocks the current thus permitting ZC turn-off. For a bi-directional switch, the active switch must be turned off while the inductor current flows negatively through the parallel diode so that the current will be blocked once it attempts to flow positively through the diode. It should be noted that for a bi-directional switch, ZV turn-off exists in addition to or in place of ZC turn-off. This occurs since the negative inductor current flows through the parallel diode effectively producing a near zero voltage drop across the active switch when the switch turns off.
Similar to ZC switching, ZV switching may be implemented using two possible switch configurations: an active switch with a passive switch in parallel, or an active switch with a passive switch in series (FIGS. 2b and 2c). These composite switches are referred to as uni-polar and bi-polar switches, respectively. A uni-polar switch has the property of permitting the capacitor voltage to be of only one polarity, whereas the bi-polar switch allows the capacitor voltage to be both positive and negative in value. ZVS converters utilizing bi-polar switches or uni-polar switches are said to operate in a full-wave or half-wave mode, respectively.
Unlike ZC switching, full-wave and half-wave modes with regards to ZV switching refer to the shape of the capacitor voltage waveform, and not the inductor current waveform. Both bi-polar and uni-polar switches may turn on passively, however, an uni-polar switch turns on passively when the capacitor voltage tries to build negatively across the switch, whereas a bi-polar switch turns on passively when the capacitor voltage tries to build positively across the switch. For a uni-polar switch, the active switch must remain off until the parallel diode conducts thus permitting ZV turn-on. For a bi-polar switch, the active switch must turn on when the capacitor voltage builds negatively across the series diode so that the switch can turn on passively when the capacitor voltage attempts to become positive. It should be noted that for a bi-polar switch, the turn-on transition definitely occurs while the current through the active switch is zero (the series diode is reverse biased,) thus guaranteeing ZC turn-on. This condition may exist in addition to, or in place of ZV turn-on.
Both ZVS and ZCS conventional techniques seek to decrease switching losses and attempt to permit high efficiency operation at higher switching frequencies. However, only ZVS truly reduces switching loss. The reason for this is that some must take place during turn-on of a ZCS switch. Parasitic capacitance across the semiconductor switch stores energy while the switch is off, and releases stored energy internally when the switch is turned on. For this reason, high frequency operation of such conventional converters, even with the attendant switching losses, is possible only with ZVS converters in actual practice.
As mentioned above, conventional switch-mode converters using ZC or ZV switching techniques have been referred to as Quasi-Resonant Converters (QRC's). QRC's are controlled differently from switch-mode converters. Load and line regulation in a QRC are achieved by varying the switching frequency of the switch rather than the pulse width. It is known that QRC's operating in a full wave mode exhibit significantly improved load regulation over those operating in a half wave mode. This is so for both ZC and ZV switching converters.
In conventional ZVS converters, full-wave operation is achieved by placing a diode in series with the active switch to allow the resonant capacitor voltage to build both positively and negatively (It is assumed that the only capacitance in the circuit is an external one placed across the composite switch comprised of the diode and the active switch). Unfortunately, parasitic capacitance exists independently across both the diode and the active switch (usually a MOSFET). The parasitic MOSFET capacitance inhibits true ZVS since the series diode turns off before all the charge in the MOSFET capacitance has had a chance to escape. As an unfortunate result, the same turn-on losses exist as in ZCS, and high frequency operation of conventional ZVC converters is not feasible.